Lopez Obrador, from the leftist Party of the Democratic Revolution (PRD), held protests that virtually paralyzed Mexico City for more than a month when he lost the last presidential election in 2006 by less than one percent and claimed fraud.

However Lopez Obrador, who lost this time by more than six points, said he is not behind the Saturday march. The event is being organized online and via flyers handed out in the street.

The PRI was synonymous with the Mexican state as it governed for 71 years until 2000 using a mixture of patronage, repression, rigged elections and bribery.

Octavio Aguilar, a senior campaign official for candidate Josefina Vazquez Mota of the ruling National Action Party (PAN), estimated that the PRI spent up to $500 million getting Pena Nieto elected, shattering the legal campaign spending limit of $30 million.

The fraud was not at the ballot box, but in the river of cash the PRI used to pay for everything from gift cards to campaign paraphernalia to years of favorable TV coverage, he said.

The Facebook page claims it was put up by a “group of Mexican citizens tired of electoral fraud and the theft and unmeasured abuse of our national resources.” Similar comments are found on Twitter under different hash tags.

The site includes plenty of pro-Lopez Obrador and anti-PRI pictures.

Students of the #YoSoy123 protest movement, which have organized several Mexico City marches in the past weeks, said they are not behind the Saturday protest.

The Mexico City metro area has a population of 21 million and is a bastion of the country’s political left, so gathering a large crowd for an anti-PRI march is hardly a challenge.

A seemingly impromptu march on July 2, one day after the election, protesting Pena Nieto’s tainted victory gathered more than 25,000 people, according to city police.

The PRI was synonymous with the Mexican state as it governed for seven decades until 2000, often using a mixture of patronage, repression, rigged elections and bribery.